If you need a commercial pattern for mid-19th century tailored items, go with Past Patterns, Galla Rock, or order a bespoke-drafted pattern from Jim Ruley or Williams Clothiers.
...or draft your own. DeVere's 1866 trouser draft is extremely simple. It requires only five measurements: Waist, seat circumference, inseam length, outseam length and front length (which is usually the same as the outseam). It can be drawn in a few minutes using graduated measures, or you can do a few simple calculations working in "common" inches. It's a great way to get into pattern drafting and will fit better than a commercial "standard size" pattern unless you measure wrong.
The results won't be graceful from the back (DeVere believed in building in enough seat room to accomodate any normal activity), but they will be period-appropriate, not an "altered modern" look.